"Send Email" indicates the step in an automation where a campaign should be sent. Automations (a.k.a. drip campaigns) MUST start with a "Send an Email", "Send SMS", or "Wait for Event" step.
As noted, an automation MUST start with either Send Email, Send SMS, or Wait for Event.
To select the kickoff email, simply click the pencil on the step and choose either a Regular or Triggered campaign that you've already created OR go ahead and create your campaign right within the Automation workflow!
For any subsequent Send Email steps, you must choose the Automated campaign type. You can make these right from your workflow. You will see all campaigns included in your specific automation on the Automation Dashboard.